How Install kernel headers on Sangoma OS

I installed FreePBX Sangoma OS on VirtualBox and tried to install Guest Additions there:

VirtualBox -> Devices -> Insert Guest Additions CD image...

the on the VM:

yum install dkms gcc kernel-devel kernel-headers -y

mkdir /media/cdrom

mount /dev/cdrom /media/cdrom

cd /media/cdrom

./VBoxLinuxAdditions.run

it returns the following error while kernel headers are already installed: 

VirtualBox Guest Additions: Kernel headers not found for target kernel 3.10.0-1127.19.1.el7.x86_64. 

This is while we have already installed kernel-devel and kernel-headers packages. This error is because the build folder in /lib/modules/$(uname -r) is missing in the default FreePBX Sangoma installation for some reason.

To fix this problem, run the following command:

ln -s /usr/src/kernels/$(uname -r) /lib/modules/$(uname -r)/build

then run 

./VBoxLinuxAdditions.run

again to build Linux Guest Additions.